The present state of the art process for the recovery of turpentine in Kraft pulp mill operation involves separation of the digester vent gas from black liquor carry over, then condensing the steam and turpentine in a condenser. The combined condensate (water and turpentine) is received in a decanter, and a noncondensible gas stream (NCG) is vented from the decanter, cooled in a gas cooler and discarded. The light sulfur compounds are thus dissolved in the hydrocarbon (turpentine) phase causing the sulfur content of the oil phase to achieve an unacceptable level for turpentine buyers.
The turpentine is separated by gravity and allowed to drain off as produced. The foul condensate, or water phase, is hydraulically displaced to a sewer, and contains various organic compounds that raise the biological oxygen demand (BOD) level of the waste water disposal ponds.
The present invention is an improvement on the prior art process. and contemplates a method whereby the foul condensate containing organic sulphides and hydrocarbons is stripped of most of these pollutants by recycling a liquid foul stream through a reconditioning drum, or gas/liquid contact vessel, e.g. a tower packed with Raschig rings or other conventional packing, that provides for good gas/liquid contact with the incoming hot vent vapor gas from the digester. The residual organic compounds in the foul stream are revaporized or azeotroped over into the recovery system of the present invention. As these compounds recycle in the foul condensate stream, they attain an equilibrium and are ultimately removed in the decanter with the main portion of the turpentine. The resultant level of contaminants in the off-gas stream is reduced from about 300-3500 ppm to a level below 40 ppm in the novel process hereof without adding prime steam to the system, thereby effecting a considerable saving in power requirements. Moreover, the present improved process does enhance the overall production of turpentine by a considerable amount e.g., as much as 3%-5%, from the foul condensate which was heretofore lost. The increase in turpentine production will allow most mills to recoup the cost of retrofitting existing equipment, as hereinafter disclosed, in a relatively short period, e.g., about 24 months. Systems currently available use stripping systems that require large volumes of costly prime steam to accomplish a similar result.